The present invention relates to a transistor power amplifier circuit including a push-pull output circuit.
Output transistors used in a transistor power amplifier circuit must be operated within an ASO (Area of Safety Operation) region so that permanent secondary breakdown is not developed in the collector/emitter voltage V.sub.CE -collector current Ic characteristics. If the operation of the transistor moves out of the ASO region, the transistor develops secondary breakdown and is permanently destroyed.
If the temperature at the collector/base junction of the transistor increases above a maximum junction temperature, thermal runaway is developed in the collector current whereby secondary breakdown develops on the transistor. The temperature at the collector/base junction of the transistor varies in proportion to the power loss at the collector/base junction.
On the other hand, the power loss of the transistor is equal to the root-mean-square value of the product of the voltage V.sub.CE across the collector and emitter and the collector current Ic. Therefore, the d-c power loss Pd(DC) of a transistor which operates on a constant collector/emitter voltage V.sub.CE and on a constant collector current Ic, is greater than the a-c power loss Pd(AC) of a transistor which operates on an instantaneous collector/emitter voltage V.sub.CE and on an instantaneous collector current Ic.
Accordingly, referring to the collector/emitter voltage V.sub.CE --collector current Ic characteristics of a transistor as shown in FIG. 4, a d-c ASO region ASO.sub.DC in which the transistor operates on a constant voltage V.sub.CE and on a constant current Ic is narrower than an a-c ASO region ASO.sub.AC in which the transistor operates on an instantaneous value V.sub.CE and on an instantaneous value Ic.
Referring to a push-pull output circuit which operates on a positive power-supply voltage Vcc and which drives a load resistor R.sub.L via an output coupling capacitor, on the other hand, it is accepted practice that two output transistors connected in series across a power-supply voltage Vcc and the ground potential GND are so biased in a d-c manner that a voltage of about Vcc/2 is applied across the collector and emitter of each of the transistors when no a-c signals are introduced.
When an output voltage swing of the above push-pull output circuit is raised and lowered responsive to the input signals with the d-c operation level of Vcc/2 as a center, and when the input signals do not cause the voltage to rise to the power-supply voltage Vcc and do not, either, cause the voltage to lower to the ground potential GND, the output of the push-pull amplifier circuit acquires a non-clipped state. Under the non-clipped output condition, the locus of operation determined by the collector/emitter voltage V.sub.CE --collector current Ic characteristics of a single output transistor does not exceed a straight line l.sub.1 connecting the two operation points ##EQU1## and [Vcc(V), O(A)], as shown in FIG. 4.
On the other hand, when excessive input signals are applied, the two output transistors are alternately rendered conductive and nonconductive responsive to the input signals. Therefore, the output of the push-pull output amplifier circuit is raised up to the power-supply voltage Vcc and is lowered to the ground potential GND responsive to the input signals, thereby to produce pulse waveforms clipped between the two voltage levels. Under the clipped condition, the two output transistors perform switching operation between the two levels, i.e., between the power-supply voltage Vcc and the ground potential GND. However, the locus of operation of a single output transistor under the clipped output condition does not exceed a straight line l.sub.2 connecting the two operation points ##EQU2## [Vcc(V), O(A)] as shown in FIG. 4.
Therefore, in the case of the a-c operation of the output transistor of the push-pull output circuit, the operation locus may fall outside the a-c ASO region ASO.sub.AC due to a variety of causes, giving rise to the occurrence of permanent secondary breakdown on the output transistor. If the base current of the output transistor is so controlled that the operation locus of the output transistor is confined within a restriction level l in the a-c ASO region ASO.sub.AC, it is possible to prevent the output transistor from being permanently broken down. For this purpose, a protecting circuit must detect any operation locus of the output transistor which has fallen outside the level l, and must further so control the base current of the output transistor that the operation locus of the output transistor is confined within the level l.
In order to increase a maximum output power of the transistor power amplifier circuit including the push-pull output circuit, on the other hand, the detection-restriction level l and the operation locus l.sub.2 under the clipped condition must be brought as close as possible to a limit line of the a-c ASO region ASO.sub.AC.
Through a study conducted by the inventors of the present invention, it has become clear that when the detection-restriction level l of the protecting circuit for protecting the output transistor and the a-c operation locus l.sub.2 under the clipped condition come very close to the limit line of the a-c ASO region ASO.sub.AC, the output transistor of the transistor power amplifier circuit is very likely to develop permanent secondary breakdown.
That is, when the output terminals of the push-pull output circuit are short-circuited in a d-c manner, the operation locus ls of the d-c short-circuited can be diagramatized in FIG. 4. Due to the detection-restriction level l of the protecting circuit, on the other hand, the d-c operation point Ps of the output transistor when the output terminals are short-circuited in a d-c manner is restricted to a point at which the detection-restriction level l meets the operation locus ls when the output terminals are short-circuited in the d-c manner, as shown in FIG. 4.
As will be considered from FIG. 4, however, the d-c operation point Ps when the output terminals are short-circuited in the d-c manner is likely to be located outside the d-c ASO region ASO.sub.DC. In such a case, the temperature at the collector/base junction of the output transistor will exceed a maximum junction temperature, giving rise to the occurrence of permanent secondary breakdown.